The invention relates to optical glasses which have refractive indexes nd of from 1.52 to 1.58, Abbe numbers vd of from 50 to 57 and negative anomalous partial dispersions xcex94Pg,F of  less than xe2x88x920.0090. These glasses are of the short flint special glass (SFS) type of optical glass.
Since in recent years the glass components PbO and As2O3 have been debated in public as environmental pollutants, the manufacturers of optical equipment also have a need for PbO-free and preferably also As2O3-free glasses having the respective optical properties.
Simple replacement of lead oxide by one or more constituents generally does not succeed in reproducing the desired optical and glass-engineering properties affected by PbO. Instead, new developments or extensive changes in the glass composition are necessary.
Parameters which are decisive for the use of an optical glass are the refractive index, for example nd, and the change in refractive index with wavelength, known as the dispersion. A term for the change in refractive index with wavelength is the Abbe number, for example       v    d    =                    n        d            -      1                      n        F            -              n        C            
The difference nFxe2x88x92nC is known as the principal dispersion. Other differences are partial dispersions. Relative partial dispersions are the ratio of a partial dispersion to the principal dispersion, for example       P          g      ,      F        =                    n        g            -              n        F                            n        F            -              n        C            
Like the Abbe number, the relative partial dispersion is an important material constant for an optical glass. The majority of glasses satisfy an approximately linear relationship between Px,y and v, according to which Px,y=ax,y+bx,yxc2x7v (standard straight line).
Glasses which do not satisfy this equation are referred to as glasses having anomalous partial dispersion. The equation must then be expanded by an additional correction term xcex94Px,y:
Px,y=ax,y+bx,yv+xcex94Px,y
Depending on whether xcex94Px,y is greater than or less than xe2x80x9c0xe2x80x9d, the glasses are then referred to as glasses having positive or negative anomalous partial dispersion. Through a suitable combination of optical glasses having different Abbe numbers, the imaging flaw, chromatic aberration, can be eliminated or at least improved in lens systems, for example for 2 colours. The residual chromatic aberration (chromatism) which remains for the uncorrected colours is referred to as the secondary spectrum. This effect is particularly disadvantageous for high-performance optics, since it impairs the imaging sharpness and resolving power of the optic. However, use of glasses having anomalous partial dispersion in optical lens systems would succeed in reducing the secondary spectrum and thus giving corrected lens systems having excellent imaging sharpness and high resolving power.
Particularly desirable is correction in the blue region of the visible spectrum, for which the relative partial dispersion Pg,F mentioned above by way of example is characteristic. In its standard straight line, ag,F=0.6438 and bg,F=xe2x88x920.001682.
The patent literature has already revealed some specifications which describe glasses having optical values nd and vd from these or adjacent regions, only some of the glasses having high negative anomalous partial dispersion xcex94Pg,F. The glasses described have a wide variety of disadvantages:
Patent Specification DD 1603 07 relates to optical crown glasses having refractive indexes ne=1.500-1.555 and Abbe numbers ve=57-62 with negative anomalous partial dispersion which have a very high B2O3 content, namely 73-87% by weight, and thus do not have adequate chemical resistance for practical purposes.
By contrast, DE-B 13 03 171 relates to a batch for the production of optical glasses having anomalous partial dispersion and an Abbe number ve=40-60 and refractive indexes ne=1.52-1.64, where the batch only contains up to 34% by weight of B2O3, but up to 40% by weight of SiO2. These glasses do not achieve the desired magnitude of the negative anomalous partial dispersion.
Like the glasses of said DD 1603 07, the glasses of DE-B 1 022 764 having negative anomalous partial dispersions and Abbe numbers of from 64 to 35 and refractive indexes of from 1.53 to 1.73 do not contain SiO2. Consequently, these glasses likewise do not have adequate chemical resistance.
JP 60-469 46 A discloses UV-transparent glasses of the borosilicate glass type which are predominantly in the quaternary system CaOxc2x7Al2O3xc2x7B2O3xc2x7SiO2 (CaO+Al2O3+B2O3+SiO2=9-100% by weight). However, with other components  less than 10% by weight, highly negative partial dispersion cannot be achieved.
German Patent Specification DE 42 18 377 C1 describes optical crown glasses having negative anomalous partial dispersion xcex94Pg,F, a refractive index nd greater than 1.52 and an Abbe number vd greater than 57. In these glasses, in which, inter alia, SiO2 at up to 15% by weight and Ta2O5 at up to 10% by weight are only optional components, a defined water content of 0.1-0.5% by weight is necessary in order to ensure sufficiently high anomaly of the partial dispersion. Such setting of a defined water content makes glassmaking undesirably complex.
JP 63-222 040 A claims glasses from a very broad composition range which varies greatly with respect to the possible components, but lists only a few specific glasses having only a few particular constituents. This specification, which relates to glasses for ultrasound retardation lines, gives, due to the completely different objective, absolutely no indication of how a lead-free glass having a refractive index nd of from 1.52 to 1.58 and an Abbe number vd of from 50 to 57 and having a highly negative anomalous partial dispersion, namely xcex94Pg,F less than xe2x88x920.0090, can be produced.
This, however, is the object of the present invention. It is achieved by the glasses described herein.
The glasses contain predominant proportions of the glass formers Al2O3, B2O3 and SiO2: they contain 8-13% by weight of Al2O3, which has a positive effect on the chemical resistance. At higher contents, however, the melting properties would be impaired and the tendency toward crystallization increased.
The glasses contain 45-55% by weight of B2O3. B2O3 dissolves components which are difficult to melt, and B2O3 lowers the negative partial dispersion xcex94Pg,F (i.e. increases its value) of the glasses. At higher contents, the chemical resistance would be impaired and the Abbe number would be too high.
The glasses contain SiO2 in contents of greater than 15% by weight and at most 25% by weight. At lower contents, the good chemical resistance drops, while at higher contents, the good melting properties are impaired. The SiO2 content helps to achieve an Abbe number in the low region desired.
CaO, present to the extent of 2-8% by weight, and optionally also ZnO (0-5% by weight), increase the refractive index and contribute to the lowering of xcex94Pg,F. At excessively high contents, the Abbe number would be too high and the chemical resistance would be impaired, and therefore the sum of CaO and ZnO should remain limited to 10% by weight.
The glasses contain from 3 to 8% by weight of alkali metal oxides, which improve the melting properties of the glasses and stabilize them against separation. The said total content can be achieved by means of 0-4% by weight of Na2O, 0-4% by weight of K2O and 0-4% by weight of Li2O. Preferably, however, at least two alkali metal oxides are present, in particular at least Na2O and K2O. It is particularly preferred for all three components to be present.
The glasses may furthermore contain up to 14% by weight of La2O3. La2O3 supports a high Abbe number at the same time as a high refractive index.
The glasses contain Ta2O5 in an amount of at least  greater than 10% by weight in order to achieve the desired high negative anomalous partial dispersion, and at most 17% by weight, since otherwise separation would occur. In addition, Ta2O5 improves the chemical resistance and increases the refractive index, as do the optional components Nb2O5 and ZrO2.
Nb2O5 can support Ta2O5 in its effect on the partial dispersion. In addition, it has positive effects on a low Abbe number. Owing to the separation which occurs at higher contents, the Nb2O5 content is restricted to a maximum of 4.8% by weight, preferably to  less than 4.8% by weight, and the sum of Nb2O5 and Ta2O5 is restricted to a maximum of 17% by weight, preferably to  less than 15% by weight.
For the same reason, the ZrO2 content is also restricted to a maximum of 7% by weight, preferably to  less than 7% by weight, the sum of ZrO2 and Ta2O5 being restricted to  less than 18% by weight. It is preferred also to restrict the sum of ZrO2, Nb2O5 and Ta2O5 to a maximum of 18% by weight.
The glasses can contain up to 2% by weight of SnO2. SnO2 is used to fine tune nd and vd, nd rising and vd falling with increasing SnO2 concentration.
In order to improve the glass quality, one or more fining agents known per se can be added to the batch in the usual amounts in order to fine the glasses. In this way, the glasses have particularly good internal glass quality with respect to freedom from bubbles and streaks.
If the fining agent used is not As2O3 or Sb2O3, the lead-free glasses according to the invention are in addition free from arsenic and antimony.
Since SnO2 also has a fining function, the presence of at least 0.1% by weight of SnO2 is preferred. These glasses, even without further fining agents, have excellent internal glass quality.
Within the composition range according to the invention, there is a preferred composition range of glasses having refractive indexes nd of from 1.52 to 1.575 and Abbe numbers vd of from 50 to 56.5. It is (in % by weight, based on oxide): SiO2 greater than 15-23.5, B2O3 45-50, Al2O3 8-11.5, CaO 2-5.5, ZnO 0- less than 5, with CaO+ZnOxe2x89xa610, Li2O 0-2.5, Na2O 1-2.5, K2O 0.5-2, with Li2O +Na2O+K2O 3-6, La2O3 0-13.5, Nb2O5 0- less than 4.8, Ta2O5 greater than 10-15, with Ta2O5+Nb2O5xe2x89xa615, SnO2 0.1-0.5, ZrO2 0- less than 7, with Ta2O5+ZrO2 less than 18.
Particular preference is given to glasses from the composition range (in % by weight, based on oxide): SiO2 greater than 15-16, B2O3 48-50, Al2O3 8-11.5, CaO 2-4, ZnO 1- less than 5, with CaO+ZnOxe2x89xa68.5, Li2O 0-2, Na2O  greater than 1-2, K2O 0.6-2, with Li2O+Na2O+K2O 3-6, Nb2O5 0-2, Ta2O5 greater than 10- less than 15, with Ta2O5+Nb2O5 less than 15, SnO2 0.1-0.4, ZrO2 1- less than 7, with Ta2O5+ZrO2 less than 18. These glasses have refractive indexes nd of from 1.54 to 1.56 and Abbe numbers vd of from 52.5 to 54.5.